1. Field of the Invention
This invention relates to fuel cells, and more particularly, to corrosion prevention in the coolant systems thereof.
2. Description of the Prior Art
In a fuel cell stack the individual fuel cells are separated by plates made of conductive material such as carbon. The plates separate the reactant gases in adjacent fuel cells from each other and are also usually electrically connected in series to each other to carry electricity between the cells. A load is connected across the stack to complete the circuit. During operation electrons flow from the negative end of the stack to the positive end of the stack through the load. There is generally a large potential drop from one end of the stack to the other made up of smaller potential drops between adjacent cells. Heat generated by the fuel cell stack is often removed by flowing water or other fluids through channels in the separator plates between the cells. These fluids are often ionically conductive. Generally the water is manifolded to pass through separator plates in parallel where it is collected in a manifold at the other side of the cells; the heat of the cell may change the water to steam which may be used in various components of a fuel cell system, or the heat adsorbed by the water may simply be radiated out to the atmosphere and the water recirculated through the stack. Because of the potential difference between the ends of the fuel cell stack and due to the manifolding of the water from one end of the stack to the other, and because the water is in contact with the current conducting and electrically connected separator plates, shunt currents flow through the water. These shunt currents cause the carbon plates nearest the positive end of the stack to corrode with time which can be a serious problem in fuel cells which must operate continuously for many thousands of hours.
One solution, called edge cooling, involves flowing the coolant at the edges of the cells only (i.e., no flow between cells). The coolant is electrically insulated from the cells and thus no shunt currents are present. This technique often results in an unacceptable temperature distribution across the stack. Another common solution is to use a dielectric coolant which cannot carry current. However, a dielectric is not as good a coolant as water and it may also be more expensive than water.